Although many heavy nuclei can be induced to fission by high energy neutrons, some can be made to split by neutrons with low kinetic energy. Isotopes that can sustain a chain reaction through fissions induced by thermal (slow) neutrons arefissile.
The heavy transuranic elements uranium and plutonium are fissile.
The important fissile isotopes are:
uranium-233 (233U): produced (bred) from thorium-232 by neutron capture;
uranium-235 (235U): naturally-occurring isotope of uranium that comprises only 0.7 % of all uranium;
plutonium-239 (239Pu): bred from uranium-238 by neutron capture; and
plutonium-241 (241Pu): bred from plutonium-240 by neutron capture.
To reiterate, uranium-235 is the only naturally-occurring fissile element and it comprises only 0.7 % of natural uranium.
Nuclear Reactions: Fission and Fusion
Fission
What is fission?
YouTube clip 41 seconds
YouTube explanation 3 minutes
Process in Nuclear Power Plants
The Element Uranium
Other isotopes such as uranium-232 have been produced in breeder reactors.
Naturally occurring uranium is composed of three major isotopes:
The Hiroshima Syndrome (on Uranium)
More on Uranium
Section below is from: http://www.gg.uwyo.edu/content/laboratory/nuclear/power/fission/fissile.asp?callNumber=14276&SubcallNumber=0&color=&unit=2
The heavy transuranic elements uranium and plutonium are fissile.